The early phone line was a pair of twisted wires, which was found to be useful and economical for transmitting the analog voice signal from the phone to its destination. These lines continue to be in wide use up to the present time, particularly for transmitting the voice signal from the phone to the local phone office on a “local loop”.
With the development of devices that generate digital signals, it became imperative to transmit the data from these devices to other digital devices.
In addition, it is economical to transmit signals from numerous sources simultaneously on one line as the demand for more transmitting capacity has grown.
The technique that has evolved has been to convert the digital signal at each origin of the signal to an analog signal by amplitude, phase or frequency modulation of a carrier wave. Up to twelve such modulated waves may be transmitted simultaneously (stacking) in one phone line
The requirement to convert a digital signal to an analog signal at the source of the signal for transmission over a phone line; and to convert an analog signal from the transmission line to a digital signal for interpretation by a digital device has been the motivation (and evolution) for developing the modem.
A modem is a circuit package connected between an end of a phone line and a source of a digital signal. The modem converts the digital signal to analog form so that the signal can be transmitted by the phone line to an intended receiver of the signal. If the receiver can process only a digital signal, then the signal must first be converted back to digital form by a second modem connected between the receiver and the other end of the phone line.
FIG. 1 (PRIOR ART) is a block diagram of a typical system showing two data terminal equipments (DTE) 10, 12 connected by a telephone line 14 through a pair of modem systems 16, 18. Each DTE, 10, 12, has a UART, shown in FIG. 2 (PRIOR ART)
The data signal is output in parallel from the DTE 10 and converted to a serial signal by the respective UART. UART. The serial signal is input to one modem 16 that converts the serial signal from a digital signal to an analog signal for transmission over the telephone line 14. The analog signal is converted back to a digital signal by the second modem and then converted to the original parallel signal by the second UART 22 for input to the second DTE 24.
The typical telephone line between the DTE and the local telephone office is a twisted pair of wires.
The transmission format of the string of bits and zeroes representing words of data is determined by the DTE and includes “marks” and “spaces”. A mark is represented as one level of voltage or current; a space is represented by another level of voltage or current. Marks and spaces are included in the stream of bits and zeroes to mark the timing of the message. The pattern of marks and spaces depends on the mode of the DTE being any one of simplex, (one way), duplex (alternate one way), synchronous (simultaneous two way). The “asynchronous” mode is characterized by marks and spaces that synchronize the signal reception by the receiving DTE with the signal transmission from the sending DTE.
The history of MODEM development has included a number of versions particularly directed toward smaller size with improved convenience.
The first MODEMs were enclosed in housings that were about the size of a cigar box. As fabrication of circuits improved, MODEMs were developed in which all of the components of the MODEM were mounted on a single board and occupied smaller space.
The demand for decreased size of the MODEM is generated by several factors.
One factor is the simple taste of the public for smaller and smaller devices. “A MODEM housing the size of a matchbox is inherently more acceptable to the buying public than a size of a cigar box.
A more urgent factor is the increasing complexity of networks where telephone connections to numerous branches requires that such connections be assembled together in as small a space as possible.
These requirements led to versions of more compact MODEMs that gained wide acceptance in the marketplace.
The current trend in miniaturization by means of very large scale integration (VLSI) has led to the development of single and multiple chip sets of integrated circuits that perform most of the functions required of a modem.
The present approach to modem design is directed toward a specialized chip set provided with all of the required modem functions.
However, in spite of the developments introduced by the most recent MODEMs, there is still a need for a smaller less imposing MODEM. The need is particularly manifested in the desire for tighter clusters of line connectors.
A restraint on the reduction of size of the MODEM is imposed by isolation that is required between components of the MODEM.
UL Standards have been established for limiting the distance between the line components and the MODEM circuit.
This requirement inherently places a restriction on the reduction in size of the modem that can be achieved by applying the present approach to Modem design.